This invention relates to medical diagnostic ultrasound systems and methods for detecting harmonic energy using Doppler processing. In particular, pulse inversion techniques are used with Doppler detection.
Imaging of small vessels may increase the potential for obtaining diagnostic information. One method for enhancing the imaging of small vasculature is the injection of contrast agents. Using B-mode imaging, the injected contrast agents may increase specificity. By isolating signals associated with a second harmonic of the fundamental transmit frequency, contrast agents may further increase the specificity for imaging small vasculature. However, many vessels, such as small vessels in the myocardium may remain undetected using these techniques.
Lost pulse-to-pulse correlation may help identify small vessels in a B-mode image of contrast agents due to visible temporal flickering, but the lost correlation can also be used in Doppler imaging to further increase contrast agent to tissue specificity. In Doppler, two or more pulses are transmitted in the same direction, and differences between the pulses are highlighted by Doppler processing. Using the Doppler frequency shift to show movement may identify moving contrast agents in small vessels. Using harmonic detection with Doppler processing (i.e. harmonic Doppler), contrast agents and vessels may be detected below the resolution limits of B-mode imaging. However, clutter associated with moving tissue may limit the resolution and detectability of small vessels.
Another technique, pulse inversion, for increasing the specificity of contrast agents for ultrasound imaging is disclosed by Hwang et al. in U.S. Pat. No. 5,706,819 (the '819 patent). Contrast agent is injected into a target. Two or more pulses are transmitted on the same scan line within the target. Each pulse is associated with a different envelope phase or polarity, such as transmitting the first pulse with the phase of zero degrees and transmitting the second pulse with a phase of 180 degrees. By adding the consecutive echo pairs associated with these transmissions, i.e., P.sup.1 +P.sup.2, P.sup.2 +P.sup.3, etc., the fundamental components are suppressed and the harmonic components of the echo signals are summed. If fundamental components are suppressed, shorter transmit pulses for increased axial resolution and finer spatial sampling of flow may be used.
In the Doppler domain, pulse inversion provides a spectrum with separated odd and even harmonics. Odd harmonic components, including the dominant fundamental response, are modulated by half the pulse repetition frequency, and echoes from the even harmonics remain unmodulated at the zero frequency. Provided that a velocity scale is chosen to minimize aliasing, such as using half the conventional limit, odd and even harmonics can be uniquely identified in the Doppler domain.
Pulse inversion Doppler techniques may increase the specificity of contrast agents, allowing for better imaging of small vessels. The ratio of non-linear response to linear response using Doppler energy may identify contrast agent with improved specificity. This contrast improvement may allow for transmissions at reduced power, providing less contrast agent destruction and possibly larger durations of agent enhancement.
In the above-discussed '819 patent, the non-linearity associated with the tissue is assumed to be negligible (column 5, line 15). However, the non-linear response from tissue can be significant. These non-linearities can introduce unwanted harmonic clutter that interferes with detection of non-clutter harmonic energy. Two pulse summation or multiple pulse summation by "accumulating the partial sum of consecutive pairs of echoes" (column 6, line 4) is disclosed. This two echo pair summation of sequentially acquired ultrasound lines provides a lowpass Doppler wall, or clutter, filter with an impulse response of [1 1]. Two echo summation limits the ability of the disclosed system to optimally suppress fundamental clutter. Pulse to pulse pair summations may suppress some fundamental clutter at, or near, plus or minus one half the PRF, but may not suppress harmonic clutter near DC. Further, the [1,1] impulse response can be ineffective for even slight movements of clutter signal sources, introducing deleterious clutter and masking the signals of interest.
In "Pulse Inversion Doppler: A New Method for Detecting Nonlinear Echoes from Microbubble Contrast Agents", 1997 IEEE International Ultrasonics Symposium, Toronto, Canada, Oct. 5-8, 1997, by D. Simpson et. al., the use of a modified wall filter is noted. Using the modified wall filter for pulse inversion with Doppler imaging provides 15 dB more agent to tissue contrast in the myocardium than for harmonic Doppler processing. In "Perfusion Imaging with Pulse Inversion Doppler and Microbubble contrast Agents: In Vivo Studies of the Myocardium", 1998 IEEE International Ultrasonics Symposium, Sendai, Miyagi, Japan, Oct. 5-8, 1998, D. Simpson et. al., suggest that such wall filters can be designed and a specific type of filter design taken from MTI Doppler RADAR theory is used for estimating energy only, not velocities or variances, before and after contrast injections. However, further modifications of the wall filters are not discussed, and the desired filtering of harmonic and fundamental clutter signals may be further optimized.